|Urban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.)|
|Chapter 2. An Introduction to Urban and Rural Wastewater Management|
2.3.1 Why have we developed a waterborne system for carrying away our waste?
In early urban settlements, waste was dealt with at individual dwellings. The quantity of water used was considerably less than today. Water would need to be drawn from wells or the nearest watercourse and carried or hauled to the dwelling. Only rarely was water piped to centres of population and even more rarely to individual houses. Water used for personal hygiene, the washing of floors and for cooking soaked away into the ground. Human excreta was at best stored and this nightsoil, as it is sometimes called, was either carted away to a tip or watercourse or, as was common in China from early times, used as a fertiliser in agriculture.
As towns grew in size, it became an onerous task to cart away nightsoil from an increasing number of houses. The Romans designed and constructed drains beneath the streets to carry the water and nightsoil mixture away by gravity to ditches and watercourses.
However, with the passing of the Roman Empire, it appears that its drainage techniques and practices fell into disuse. Although a few towns could be said to have systematised their water supply and wastewater infrastructure throughout the ages, in general, most towns and cities grew unplanned and lacked any form of system to carry away waste. Where a system was provided, it generally consisted of open ditches along the centre of streets into which all manner of domestic waste was thrown. These ditches were occasionally cleaned by the municipal authorities but more often than not were left to fill and fester until a storm carried away the accumulated mess. Open, natural watercourses collected the waste from the streets and when the flow in them was sufficient, the wastes were carried away from the city. Crowded cities stank from these practices and periodic bouts of dysentery and bubonic plague, which decimated medieval urban populations from time to time, were a natural consequence of such unhygienic practices.
2.3.2 So what stimulated the invention and construction of our modern system of sewers to carry away the wastewater that we produce?
With the advent of the industrial revolution in Europe in the late 1700s, there was a considerable migration of the rural population to the towns that grew rapidly in size. Local wells and handpumps could no longer cope with the demand for water and, in addition, human waste seeped into the aquifers and contaminated the water withdrawn from wells for local use. The invention of steam-driven pumps enabled clean water in considerable quantities to be brought to the cities from sources remote from centres of population. Increasingly, a clean and abundant water supply was made available to cities and towns, initially at public standpipes in the streets and gradually, through a system of pipes, direct into houses and factories.
Fig 2.1 Small diameter sewer under construction
Fig. 2.2 Effluent-producing factory
From the early 1800s, it became necessary to construct a system of drains to carry away such large quantities of water if the streets of towns and cities were not to become continually awash. Industrialised production of the water-closet at about this time made its installation in houses more common and further increased the pressure for sewer systems to be constructed. Quite naturally, drains constructed to carry away water used for laundering and for personal hygiene were also used to carry away water closet waste.
Although existing open watercourses and storm drains had been used for some time to carry away waste, these were now culverted, roofed over, and additions to the system were constructed as closed culverts from the outset. In time, sewer systems were designed to comprehensively serve all development and where a gravity system could not cope alone, pumping stations were constructed.
At first, wastewater was carried away to the edge of developments and discharged untreated into watercourses, lakes and the sea. As wastewater quantities increased and the link between sewage, disease and hygiene became firmly established in the middle of the 19th century, conditions in rivers, lakes and coastal waters became aesthetically and hygienically unacceptable. In order to reduce the polluting potential of the wastewater, treatment plants were constructed from about this time. Initially sewage was either subjected to settlement or conveyed to farmland on the outskirts of towns and cities. So common and important was this practice that the public still often refer to modern, sophisticated treatment plants as sewage farms.
Treatment techniques have now been developed to the point that, if necessary, under extreme circumstances, wastewater can be treated and reused as drinking water. However, under most circumstances, it is neither environmentally necessary nor economic to treat to such a high standard.
2.3.3 What does an urban wastewater system consist of?
An urban wastewater system is composed of a sewer system, a wastewater treatment works and an effluent discharge pipe. Sewer systems are described in Chapter 5 and wastewater treatment works in Chapters 6 and 7.